The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for reducing local specific absorption rate (“SAR”) in a subject imaged with MRI.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a radio frequency (“RF”) excitation field, B1, that is in the x-y plane and that includes a frequency component near the Larmor frequency, the net aligned moment, Mz, of the nuclei may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment, Mxy. An “MR” signal is emitted by the excited nuclei or “spins,” after the RF excitation field, B1, is terminated, and this signal may be received and processed to form an image.
In producing an image from the emitted MR signals, the MR signals are spatially encoded using magnetic field gradients (Gx, Gy, and Gz) so that the detected MR signals can be attributed to the appropriate locations in an image. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct an image using one of many well known reconstruction techniques.
The success of parallel reconstruction methods and their impact on image encoding has sparked a great deal of interest in using the spatial distribution of RF transmit coils in an analogous fashion commonly referred to as “parallel transmit,” “parallel transmission,” or simply “pTx.” For example, by breaking down the RF transmit field into multiple regions that are each controlled by a separate transmit channel, spatial degrees of freedom are created that allow the spatial information in the array to be exploited in the excitation process. Anatomy-specific excitations could potentially reduce image encoding needs, such as for cardiac or shoulder imaging, by reducing the required field-of-view; allow selective spin-tagging excitations, thereby potentially allowing vessel territory perfusion imaging; or simply provide clinically useful, but nontraditional, excitations, such as curved saturation bands for the spine or brain.
While the development of these novel RF pulse designs and applications continue to be an area of intense development, the clinical utility of a given RF excitation pulse is characterized by more than just its spatial fidelity. Particularly, the specific absorption rate (“SAR”) of an RF excitation pulse is often the critical limiting factor when applied to a clinical imaging sequence because SAR. For a discussion of the regulatory concerns on SAR in the United States, see, for example, Center for Devices and Radiologic Health “Guidance for the Submission of Premarket Notifications for Magnetic Resonance Diagnostic Devices,” Rockville, Md.: Food and Drug Administration, 1998; and in Europe, see, for example, International Electrotechnical Commission, “International Standard, Medical Equipment-Part 2: Particular Requirements for the Safety of Magnetic Resonance Equipment for Medical Diagnosis, 2nd Revision,” Geneva: International Electrotechnical Commission, 2002.
The need to stay below safe SAR limits often requires unfavorable tradeoffs in acquisition parameters such as increased repetition time (“TR”) or reduced flip angle. SAR becomes especially problematic at field strengths of 3 Tesla, where the power needed for a given flip angle increases as much as four-fold as compared to 1.5 Tesla applications. Regulatory limits on SAR rely on two different parameters: the global average power deposited in a given body part, and the local SAR deposited in any ten grams of tissue. Often, global power must be reduced to insure that an imaging session remains within the local SAR limits. Therefore, it would be beneficial to provide a method for reducing local SAR without undue tradeoffs to other imaging parameters.
One existing solution to the local SAR problem in parallel transmit penalizes local SAR during the design of RF excitation pulses. Such an approach has proved to be computationally difficult because the location of the local SAR hotspot can be anywhere in the body. Attempts to make local SAR penalization methods more computationally feasible have included incorporating image compression methods into the RF excitation pulse design, while other methods have attempted to make the local SAR hotspots change position with time by using different excitations for different k-space lines.
Another proposed solution to the local SAR problem is to employ slightly different gradient trajectories for each pulse in order to force different local SAR patterns. For example, a slice-selective spoke-trajectory excitation could be generated from any one of hundreds of k-space spoke trajectories. Combing through these solutions produces dozens of candidates within a given spatial fidelity and with different local SAR patterns. An even more sophisticated formalism computes the average SAR pattern from the collection of pulses and optimizes the individual pulses based on this time-averaged pattern.
In light of the foregoing, it would be desirable to provide a method for reducing local SAR in parallel transmit MRI that is less computationally intensive than currently existing methods. It would also be desirable to provide a method for reducing local SAR that includes reducing the influence of local SAR hotspots.